It is known that an unsaturated monomer or an unsaturated polymer (prepolymer) is placed together with a suitable photopolymerization initiator and polymerized by light irradiation. Such photopolymerizable compositions are used in various fields, for example, in lithographic, relief or intaglio printing plates, light-curable inks, coating materials, photoresists for printed circuit boards, adhesives, etc.
Particularly in some applications, reduction of the time of exposure to actinic rays is required, and for this purpose, various attempts have heretofore been made to increase the polymerization rate of a photopolymerizable composition, namely, to increase its photosensitivity.
For example, a method comprising adding a photopolymerization initiator or a sensitizer to a photopolymerizable composition containing an ethylenic unsaturated group containing compound is employed for increasing the photosensitivity. As the photopolymerization initiator or the sensitizer, there are used, for example, polynuclear quinones such as 2-ethylanthraquinone, 2-t-butylanthraquinone, etc.; aromatic ketones such as benzophenone, 4,4'-bis(dimethylamino)benzophenone, etc.; and benzoin derivatives such as benzoin methyl ether, benzoin ethyl ether, etc.
However, these aromatic compounds are disadvantageous in that they cause coloring of cured products and also in that since they are not sufficiently effective, light irradiation for a long period of time is required. Therefore, they have not been satisfactory for practical purposes.
In order to increase the photosensitivity, there have been disclosed employment of a combination of an aromatic ketone such as aminophenyl ketone and a 2,4,5-triarylimidazole dimer (Japanese Patent Publication No. 48-38403 and U.S. Pat. No. 731,733) and employment of a combination of aminophenyl ketone and an active methylene compound or an amino compound (Japanese Patent Publication No. 49-11936 and U.S. Pat. No. 877,853), but they are still not satisfactory.
On the other hand, 9-substituted acridine compounds are proposed as highly photosensitive compounds in Jap. Pat. Appln. Kokai (Laid-Open) No. 47-4126, but are not sufficiently effective. Acridine or phenazine compounds which can contain a fused benzole ring are disclosed in Japanese Patent Publication No. 53-27605 as photopolymerization initiators which have high sensitivity and storage stability particularly in the presence of oxygen, but these compounds are still not satisfactory.
A combination of 9-phenylacridine and a thiol group containing heterocyclic compound is disclosed as a highly sensitive photopolymerization initiator. But, the above combination does not bring about a sufficient improving effect.
Substituted 9-benzoylacridines are disclosed as highly sensitive photopolymerization initiators in Jap. Pat. Appln. Kokai (Laid-Open) No. 60-164739.
At present, however, no photopolymerizable composition having a sufficient photosensitivity can be obtained even by use of the compounds described above.